1. Field of the Invention
The present invention relates generally to plasma wave tubes.
2. Description of the Related Art
Plasma wave tubes typically inject high-power electron beams into a waveguide filled with an ionizable gas to first ionize the gas into a plasma, and then generate electron plasma waves (EPWs) in that plasma. If the electron beam power is sufficiently high, the EPWs interact to generate electromagnetic energy which may be coupled out of the tube as an RF signal. Plasma wave tubes generally do not require externally applied magnetic fields for control and transport of the electron beam because the beam space charge is canceled by the generated plasma.
An exemplary plasma wave tube is described in U.S. Pat. No. 4,916,361 assigned to Hughes Aircraft Company, the assignee of the present invention. The structure disclosed in that patent injected two opposed electron beams into an ionizable gas to create the interacting EPWs.
Another exemplary plasma wave tube is described in U.S. Pat. No. 4,912,367 which is also assigned to Hughes Aircraft Company. This patent disclosed the injection of a single high-current electron beam into a smooth cylindrical waveguide to modify the plasma density in a periodic fashion which, in turn, produced backscattered plasma waves. The result, in effect, was a pair of counterpropagating plasma waves which coupled nonlinearly within the plasma to generate electromagnetic radiation.
A theory directed to the signal generation processes of plasma wave tubes that use counterpropagating electron beams was described in Santoru, Joseph, et al., "Microwave/millimeter-wave generation in a counterstreaming-beam-plasma system", Journal of Applied Physics, 74 (5), September, 1993, pp. 3057-3060.
Many of the plasma wave tubes described in the patents referred to above generally operated as free running oscillators whose output frequency was typically adjusted indirectly by changing the plasma density and, hence, the plasma frequency. An amplifier embodiment was also described in U.S. Pat. No. 4,916,361 in which a coherent microwave input signal was directed to intersect the opposed electron beams in an orthogonal relationship to provide a phase locked output.
Prebunching of the electron beam can provide a more direct control of the EPW spectrum and, hence, the output radiation frequency and bandwidth. However, structures that enable such prebunching in high-current, high-voltage electron beams in plasma wave tubes have proven to be difficult to realize.